Quenching rate laser flash photolysis

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

A mixture of 2-naphthoylazide and sensitizer 2-isopropylthioxanthone ( r = 65 kcal/mol) was irradiated (k > 385 nm), conditions under which the sensitizer absorbed >95% of the light. Laser flash photolysis experiments demonstrated that the triplet sensitizer is quenched by 2-naphthoylazide at a diffusion controlled rate. 

Upon laser flash photolysis of a cyclohexane solution containing DTT and DNB, no transient absorption band ascribable to the products of electron-transfer reactions was observed. The transient absorption due to t(326) disappeared within 500 ns after laser excitation, and a new absorption band appeared at wavelengths <350 nm. The new band can be attributed to the triplet excited state of DNB. The result indicates that t(326) is quenched by energy transfer to DNB. The decay rate constant of t(326) for the energy transfer was estimated to be 8.8 x 109 M 1 s. As for cyclohexane solution of 327 and DNB, quenching of T(327) by energy transfer was also observed and the rate constant was 1.1 x 1010 M-1 s... 

The high value for the quenching of 3,4-dimethoxyacetophenone by phenol suggests that it is probable that within the lignin structure hydroxyl groups are able to quench carbonyls by a static mechanism to yield phenoxy-ketyl radical pairs which decay on a timescales faster than the time resolution of our laser flash photolysis apparatus. Intersystem crossing rate constants for triplet radical pairs in the restricted environments of micelles have been demonstrated to be of the order of 2 -5 x 106 s-1 (25, 24). However, in the lignin matrix where diffusional processes are likely to be... 

Photo-induced electron transfer between [Ru(bpy)3]2+-like centres covalently bound to positively-charged polymers (N-ethylated copolymers of vinylpyridine and [Ru(bpy)2(MVbpy)]2+) and viologens or Fe (III) has been studied using laser flash photolysis techniques. It is found that the backbone affects the rates of excited state quenching, the cage escape yield, and the back electron transfer rate because of both electrostatic and hydrophobic interactions. The effect of ionic strength on the reactions has been studied. Data on the electron transfer reactions of [Ru(bpy)3]2+ bound electrostatically or covalently to polystyrenesulphonate are also presented. 

Kinetic analysis of the oxidative quenching of Ru(bipy)2+ at different MV2+ concentrations by laser flash photolysis yields the rate constants for the forward and reverse electron transfer as 5.0 x 108M-1s-1 and 2.4 x 109M-1s- respectively. The cage escape yield of the redox products in the quenching process in water is about 30%. The back reaction in Eq. (7.6) can be prevented by adding a third component such as TEOA, which is capable of reducing Ru(bipy)3+ to the 2 + state ... 

Since the lowest triplet state of nitrostilbenes at room temperature can be probed by laser flash photolysis, quenching rate constants could be measured directly [188,200], Values for kq are close to the diffusion-controlled limit (Table 17). Comparable results have been obtained for naphthyl-[410] and 2-anthrylethylenes [433] and for ADBs [33, 143, 145-147, 232, 411], For substituted 1-NPEs, conclusions about the position of the triplet equilibrium were drawn on the basis of rate constants for quenching by ferrocene and oxygen [441]. 

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